Thermoplastic elastomers are polymeric materials that behave in some ways like thermoplastics and in other ways like elastomers. They behave like thermoplastics in that above their softening point they may be processed using ordinary plastics processing equipment. For example, they may be formed by thermoplastic injection molding, extrusion, blow molding, or vacuum forming. On the other hand, when utilized below their softening point, they behave like elastomers, i.e., they exhibit the properties normally associated with vulcanized rubbers without having been subjected to vulcanization. Thus, such polymers have the elastic and resilient properties of rubber but may be processed and reprocessed like ordinary thermoplastics.
Thermoplastic elastomers have been used in coating processes when applied from a solution of the thermoplastic elastomer. Application of coating materials by solution techniques, however, involves disadvantages such as environmental problems, handling problems, as well as the problems associated with the cost and removal of solvent from the coated article. The problems encountered using solution application techniques may be avoided by applying the polymer in the form of a powder, thus eliminating the need for solvent and the various problems associated therewith. Processes for preparing powder particles are well known to those skilled in the art. See, e.g., U.S. Pat. Nos. 2,999,788 and 3,737,400 which describe processes for preparing powder particles by dissolving the polymer in a solvent and then precipitating the polymer by adding the solution of polymer and solvent to a non-solvent for the polymer.
Small average diameter, i.e., less than about 150 microns, powder particles are especially useful in coating applications such as the application of a thin coating of powder particles to glass bottles to prevent shattering of the glass. It is difficult to prepare thermoplastic elastomers in fine powder form suitable for powder coating techniques such as electrostatic spray or fluidized bed applications. Grinding the thermoplastic elastomer does not produce a fine powder even when done at cryogenic temperatures such as under liquid nitrogen (-196.degree. C.). Thermoplastic elastomers do not produce fine powder particles when precipitated from solution such as by the process described in the above-identified United States patents since the solvent tends to swell the polymers sufficiently to cause the thermoplastic elastomer to coalesce or form large particles upon precipitation.
Thermoplastic elastomer powder particles have been prepared by emulsifying rubber solutions in water with an amine emulsifier and precipitating the emulsion. This process is described in U.S. Pat No. 3,923,707.
The search has continued for improved processes for preparing powder particles of thermoplastic elastomer polymers. This invention was made as a result of that search.